### Introduction In this second part of the `FundMe` section, we have covered the majority of Solidity basics, including special functions, custom errors, immutable variables, modifiers, constructors, arrays, for loops, libraries, and much more. ### Special Functions We have encountered the special functions `receive`, `fallback`, and `constructor`. These functions do not require the `function` keyword before their name. The `receive` function is triggered when Ether is sent to a contract and the **data** field is empty. The `fallback` function is triggered when data is sent with a transaction, but no matching function is found. ### Saving Gas To save gas, Solidity provides keywords like `constant` and `immutable` for variables that can only be set once: ```solidity uint constant minimumUSD = 50 * 1e18; ``` In this example, `minimumUSD` is a constant and cannot be changed, saving gas. Unlike `constant`, which is set at compile time, `immutable` allows a variable to be assigned once during deployment. Attempts to change either `constant` or `immutable` variables will result in a compilation error. ### Sending Ether Remix provides an easy method to send Ether to a contract. After deploying the contract, you can press the `transact` button, set the transaction's value, and omit the call data. If no call data is included, the `receive` function, if exists, will be triggered. Otherwise will be executed the `fallback` function. ### Conclusion In the next section, we will move from Remix to a code editor to experiment with more advanced Solidity features. We will explore enums, events, try-catch, function selectors, abi.encode, hashing, Yul, and assembly. ### 🧑‍💻 Test yourself 1. 🏆 Attempt to answer all the theoretical questions from lessons 13 through 25, and then go back again to complete all the coding tasks.